1. Field of the Invention
The present invention relates to a method for removing a jacket of a drilling or production rig using a vessel provided with a deck. The invention also relates to a vessel provided with a deck and a device for removing a jacket of a drilling or production rig.
2. Description of the Related Art
An immobile drilling or production rig comprises a superstructure and a jacket by means of which the rig is anchored in the sea or ocean bed in which it is situated. After the rig has completed its task, it will have to be disassembled and removed in accordance with international regulations. After the superstructure has been removed from the rig, the jacket anchored in the bottom remains behind. At the moment this jacket, which normally consists of 4-10 legs and a framework arranged between the legs, is cut into smaller, manageable pieces under water. The pieces are then lifted out of the water using a normal crane arranged on a vessel. The drawbacks hereof are that it is a time-consuming and costly process. In addition, performing cutting operations, particularly in the case of such a colossal jacket, is not wholly without risk. Finally, the hoisting capacity of known ship""s hoists is limited, whereby the whole removal process is limited to hoisting pieces of a jacket of a determined weight and/or dimension.
The object of the present invention is to provide a method for removing a jacket of a drilling or production rig, wherein the above-stated drawbacks are obviated. Another object of the invention is to provide a vessel provided with a deck and a device for removing a jacket of a drilling or production rig, wherein the above-stated drawbacks are obviated.
For this purpose a method is provided according to the present invention for removing a jacket of a drilling or production rig using a vessel provided with a deck, comprising of manoeuvring the vessel close to the jacket, hoisting the jacket and tilting the jacket relative to a tilting point located close to an edge of the deck of the vessel until the jacket lies on the deck, wherein the side of the jacket directed toward the vessel is supported during the tilting. It is hereby possible to remove the jacket in one operation and carry it away using the vessel. Another advantage of the method according to the invention is that the jacket only has to be hoisted to a limited height. A further advantage is that since the jacket is removed in its entirety it is possible to re-use this jacket. The jacket can however also be disassembled into smaller pieces on the deck of the vessel, this being easier to carry out than under water, as is done according to the method known from the prior art.
The jacket is preferably supported non-slidably relative to the tilting point during tilting. The jacket is thus tilted relative to a fixed point until it lies on the deck.
In order to have the tilting progress in controllable manner the centre of gravity of the jacket and elements connected tiltably thereto is preferably located during the tilting on the side of the tilting point remote from the vessel.
After tilting the jacket is preferably shifted further onto the deck so as to enable cutting thereof into smaller pieces or to enable shipping to another location for disassembly or re-use.
In addition, a vessel is provided according to the present invention which is provided with a deck and a device for removing a jacket of a drilling or production rig, wherein the device comprises:
at least one support beam which is tiltable transversely of an edge of the deck of the vessel,
hoisting means connected to at least one support beam for hoisting the jacket,
tilting means for tilting the or each support beam relative to the vessel, and
support means for supporting the jacket which are provided on the side of the support beam remote from the vessel.
Using this vessel a jacket of a drilling or production rig that has to be disassembled and removed after use thereof can be taken out of the water in its entirety and be placed on the deck of the vessel. Using the device for removing the jacket the hoisting height is kept as limited as possible, and a relatively simple and inexpensive construction is thereby obtained which is capable of hoisting even the largest jackets of roughly 20,000 tonnes.
The hoisting means preferably comprise one or more hoisting cables and one or more cable jacks which engage on the or each hoisting cable. The piston of such a cable jack is alternately retracted and extended. Wedge-shaped clamping means automatically clamp the hoisting cable(s) and pull them through the cable jack when the piston is extended, and then lock the hoisting cable(s) in a new position when the piston retracts to its starting position. The cable jacks hoist one or more hoisting cables through a distance substantially equal to the stroke of the piston. Such a cable jack is for instance shown in the U.S. Pat. No. 5,083,469. The advantage of using cable jacks as hoisting means is that they are relatively simple, light and inexpensive.
In a preferred embodiment a projecting member is arranged pivotally on the end of the support beam lying uppermost during hoisting, and the hoisting means comprise at least one jack for the projecting member arranged between the support beam and the end of the projecting member remote from the support beam. The jack for the projecting member can be used to hoist the jacket over a relatively small distance for a relatively short time, this being necessary to xe2x80x9cpull loosexe2x80x9d the jacket from the sea or ocean bed. In addition, the hoisting cable(s) can be tightened using such a jack for the projecting member in order to develop lifting power.
The device, and preferably the or each jack for the projecting member, is provided with a swell-compensating mechanism. If the device for removing a jacket arranged on the vessel is coupled via the hoisting means to the jacket still anchored in the seabed, a compensation is required for displacements of the vessel relative to the jacket resulting from waves.
The tilting means preferably comprise a winch placed on the deck of the vessel and one or more winch cables arranged between the winch and the support beam. Using these tilting means the support beam and the jacket supported on the side of the support beam remote from the vessel can be tilted transversely of the edge of the deck.
In a preferred embodiment there are provided shifting means for shifting at least the jacket further onto the deck. After the hoisting and tilting the jacket can be shifted by means hereof to a desired position on the deck to be further disassembled or shipped to another location for disassembly or re-use.
The shifting means preferably comprise the tilting means. In a particularly advantageous embodiment the shifting means, which shift the jacket further onto the deck, also serve to tilt the support beam, whereby a saving in material, and thus weight, of the removing device is obtained.
The support means are preferably also displaceable transversely of the support beam in order to likewise enable good support of jackets with downward diverging legs during the tilting.
In order enable placing of the support beam in a practically upright hoisting position, it is provided with a ballast weight.
The ballast weight is preferably adjustable so as to be able to influence the position of the centre of gravity of the jacket and the support beam, which forms an element connected tiltably to the jacket.
In an advantageous embodiment the device for removing a jacket of a drilling or production rig comprises two support beams, which are each mounted pivotally on the edge of the deck of the vessel, and at least the one support beam is displaceable relative to the other along the edge of the vessel. By means of this device different jackets with 4-10 legs can be removed, wherein the distance between the support beams can be adjusted subject to the dimension of the jacket.
The pivotal mounting is herein preferably releasable in order to enable shifting of the tilted jacket with support beam further onto the deck.